Capillary characterization of fibrous reinforcement and optimization of injection strategy in resin transfer molding

During composite manufacturing, minimizing the residual void content is a key issue to ensure optimal mechanical performance of final products. For injection processes such as Resin Transfer Molding (RTM), the impregnation velocity has a direct impact on void creation at the flow front by mechanical entrapment of air bubbles. Previous work proposed to study capillary imbibition in fibrous reinforcement to determine optimal filling conditions during practical manufacturing. The objective of this study is to investigate further this possibility. For that purpose, an improved experimental procedure is proposed to estimate the optimal impregnation velocity from capillary rise tests and understand its effect in parts of varying geometry. Capillary rise experiments were carried out with an enhanced experimental protocol, and a new post processing technique was evaluated to analyze the results. The position of the capillary flow front was then used to deduce the optimal impregnation velocity range based on the Lucas-Washburn flow model. A series of injections were also carried out with a laboratory scale RTM mold to study the influence of flow velocity on the residual void content. Results show that the prediction from capillary characterization is close to the optimal velocity value deduced from manufacturing experiments. The study also highlights the importance of void transport during processing and suggests that the injection strategy (i.e., flow rate history) and the mold configuration (i.e., divergent versus convergent flow) are important process parameters that may influence void content and cycle time.